1. Field of the Invention
The present invention relates to a heat-sensitive recording sheet and more particularly, to a heat-sensitive recording sheet improved in color formability and sticking resistance and excellent in image stabilities such as plasticizer resistance and water resistance and in printability.
2. Related Art
Recently, heat-sensitive recording sheets which form a color by application of heat are widely used for facsimile, word processors, plotter systems, labeling and the like. This is because the printing system comprises forming color images by an instantaneous chemical reaction with heating and has the advantages such as use of small devices, high recording speed, little noise and environmental pollution, and low cost. However, in the case of customarily employed heat-sensitive recording systems which use dye precursors and electron-accepting compounds as color forming components, the white portions of the recording sheet sometimes develop color with organic solvents such as toluene and methyl ethyl ketone, and decoloration of the color formed portions sometimes occurs when the sheet comes to contact with a hair dressing, dibutyl phthalate or dioctyl phtahalate used as a plasticizer in a rubber eraser or synthetic resin film, or even when the sheet is wetted with water or the wetted portion is rubbed. Further problems are discoloration of the color formed portions or color formation of the white portions when letters, figures and the like are written with a maker or fluorescent marker, sticking of a thermal head to heat-sensitive recording layer of the sheet when it is printed, and incomplete color development due to piling on the thermal head surface. Furthermore, when the heat-sensitive recording sheet is applied for labels, tickets or forms processed by an automatic vendor or cash dispenser machine, printability of the heat-sensitive layer surface, e.g. sufficient surface strength, is required since that surface is usually printed by a press.
For improving the storage stability against organic solvents and chemicals, it has been proposed to use a bisphenolsulfone compound in place of a conventional electron-accepting compound or to use an another color forming system that makes use of an aromatic isocyanate and an imino compounds in combination (Japanese Patent Application Kokai Nos. 58-38733 and 58-104959). 2,4'-dihydroxydiphenyl sulfone (Japanese Patent Application Kokai No. 63-39991), which proved to provides particularly excellent image preservation stability, but failed to provide sufficient water resistance. Said Japanese Patent Application Kokai Nos. 58-38733 and 58-104959 showed a promise in providing both the image preservation stability and water resistance, but failed to retain records when they are wet and rubbed.
Furthermore, there have been made various proposals to solve the above defects by providing a protective layer mainly composed of water-soluble polymers, water-dispersible polymers or the like on the heat-sensitive recording layer (Japanese Patent Application Kokai Nos.55-95593, 57-188392, 58-122892, 59-1797, 59-45191, 60-115484 and 62-42884). However, even when such protective layer of water-soluble polymer or the like is provided, no sufficient chemical resistance can be obtained unless the protective layer is provided in an appreciable amount, which in turn blocks thermal color development and the resulting records tend to be poor in legibility. As the protective layer is hydrophilic, water or wet-rub resistance is not sufficient even though chemical resistance can be improved; moreover, printability is not satisfactory and thermal color developing performance on that layer tends to be poor after printing.
When heat-sensitive recording sheets having such protective layer is put on an offset or other press, picking occurs unless the layer has a sufficient surface strength characteristics, and the picking triggers blanket piling to lower workability and efficiency.